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Commander
- Joined
- May 31, 2002
- Messages
- 2,842
Re: Why all the concern with top speed?
There seems to be a lot of misconceptions that propping your boat for max RPM and speed makes it most efficient at any RPM. This is no more true on a boat than it is on a car, truck, or airplane. That's why they have multiple gears on road vehicles and variable pitch props or transmissions on the most efficient airplanes or boats. The slipping of the driving element (water for a boat, air for an airplane) allows a fixed-pitch propeller to work acceptably at most speeds.
You?ll see many high performance street cars with 5 or 6 speed transmissions that reach max speed in 4th gear (typically 1:1 transmission gearing). They are slower in 5th or 6th which are generally overdrive ratios (tranny output shaft turning faster than the engine). But if you are cruising down the freeway at 70 they are much more efficient (better MPG) in 5th or 6th gear. Same thing holds true for heavy trucks. If you?re looking for top speed (why????
) they?ll be faster a few gears down from the top where the peak of the "available power" curve crosses the "required power" curve . See this thread if you're not familiar with "power required" versus "power available" curves:
http://forums.iboats.com/showthread.php?t=225803&highlight=power
If you?re looking for acceleration you want to be in the lower gears. For max economy you want to be in the tallest gear it can pull without ?lugging? the motor.
Maximizing your speed and RPM on your boat gives you good ?all-around? performance. Kind of like 4th gear in the car example above. But if you?re looking for more acceleration (pull up skiers, etc) then you could possibly want a lower-pitched prop ? equivalent to 2nd or 3rd gear on the car. You give up a little top speed for more torque at the prop. Conversely, if you?re going to be cruising all day, you may want a higher pitched prop, equivalent to the 5th or 6th gear overdrive in a car. Again, you would give up a little top speed because the engine doesn?t have the power to push the ?taller? gear ratio as fast, but you would gain efficiency (MPG) at cruising speeds. Same reason more expensive airplanes have variable-pitch props. Low pitch for take-off and climb, high pitch for cruise. Top speed will not be at the highest pitch.
It is possible to go too far in either direction and hurt your engine. Too low a prop pitch allows it to easily over-rev. Too high a prop pitch forces the engine to be ?lugging? at low speed conditions. Since many people can?t tell when an engine is ?lugging? (it?s actually kind of hard to define) it?s safest for the manufacturers to prop the engine so it can reach max speed at max RPM. This gives decent performance throughout the RPM range without risking over-revving or lugging the engine. It's a good compromise for most conditions, but optimized ONLY for maximum speed. You're giving up some acceleration and some efficiency at lower speeds. The added complexity and cost of variable pitch props and transmissions on boats are not cost-effective for the small gains you would show in acceleration and economy in most cases.
There seems to be a lot of misconceptions that propping your boat for max RPM and speed makes it most efficient at any RPM. This is no more true on a boat than it is on a car, truck, or airplane. That's why they have multiple gears on road vehicles and variable pitch props or transmissions on the most efficient airplanes or boats. The slipping of the driving element (water for a boat, air for an airplane) allows a fixed-pitch propeller to work acceptably at most speeds.
You?ll see many high performance street cars with 5 or 6 speed transmissions that reach max speed in 4th gear (typically 1:1 transmission gearing). They are slower in 5th or 6th which are generally overdrive ratios (tranny output shaft turning faster than the engine). But if you are cruising down the freeway at 70 they are much more efficient (better MPG) in 5th or 6th gear. Same thing holds true for heavy trucks. If you?re looking for top speed (why????
) they?ll be faster a few gears down from the top where the peak of the "available power" curve crosses the "required power" curve . See this thread if you're not familiar with "power required" versus "power available" curves:http://forums.iboats.com/showthread.php?t=225803&highlight=power
If you?re looking for acceleration you want to be in the lower gears. For max economy you want to be in the tallest gear it can pull without ?lugging? the motor.
Maximizing your speed and RPM on your boat gives you good ?all-around? performance. Kind of like 4th gear in the car example above. But if you?re looking for more acceleration (pull up skiers, etc) then you could possibly want a lower-pitched prop ? equivalent to 2nd or 3rd gear on the car. You give up a little top speed for more torque at the prop. Conversely, if you?re going to be cruising all day, you may want a higher pitched prop, equivalent to the 5th or 6th gear overdrive in a car. Again, you would give up a little top speed because the engine doesn?t have the power to push the ?taller? gear ratio as fast, but you would gain efficiency (MPG) at cruising speeds. Same reason more expensive airplanes have variable-pitch props. Low pitch for take-off and climb, high pitch for cruise. Top speed will not be at the highest pitch.
It is possible to go too far in either direction and hurt your engine. Too low a prop pitch allows it to easily over-rev. Too high a prop pitch forces the engine to be ?lugging? at low speed conditions. Since many people can?t tell when an engine is ?lugging? (it?s actually kind of hard to define) it?s safest for the manufacturers to prop the engine so it can reach max speed at max RPM. This gives decent performance throughout the RPM range without risking over-revving or lugging the engine. It's a good compromise for most conditions, but optimized ONLY for maximum speed. You're giving up some acceleration and some efficiency at lower speeds. The added complexity and cost of variable pitch props and transmissions on boats are not cost-effective for the small gains you would show in acceleration and economy in most cases.
